Reclassify .toml files as .c files on GitHub

With the new test framework, GitHub really wants to mark littlefs as a
python project. telling it to reclassify our test .toml files as C code
(which they are 95% of anyways) remedies this.

An alternative would be to add syntax=c vim modelines to the test/bench
files, which would also render them with C syntax highlighting on
GitHub. Unfortunately the interspersed toml metadata mucks this up,
making the result not very useful.

.gitattributes

@@ -0,0 +1,4 @@
+# GitHub really wants to mark littlefs as a python project, telling it to
+# reclassify our test .toml files as C code (which they are 95% of anyways)
+# remedies this
+*.toml linguist-language=c